Summary

In this lab, students will examine the reaction between citric acid and baking soda. They will analyze the chemical equation, balance it and calculate needed quantities of each reactant for a complete reaction. Based on their observations, students will determine if all reactants were completely used during the reaction.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Objectives

By the end of this lab, students should be able to

• Use and apply stoichiometric concepts to determine the ratio of moles in the reaction.
• Understand solubility rules in order to determine if a reactant is present at the end of a chemical reaction.
• Determine factors affecting the rates of chemical reactions.

Chemistry Topics

This lab supports students’ understanding of

• Chemical Reactions
• Stoichiometry
• Balancing Equations
• Rates of Reaction
• Solubility

Time

Teacher Preparation: 30 minutes

Lesson: 1 hour

Materials

Per lab group:

• Analytical balance
• 20g citric acid (H₃C₆H₅O₇)
• 26g baking soda (NaHCO₃)
• Weighing paper or weighing dish
• 50 mL beaker
• Deionized water/distilled water
• 25mL graduated cylinder
• Periodic Table

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• Neutralize any acid spills with baking soda.
• Wear protective gloves when working with strong acids.

Teacher Notes

• Students can work in pairs or in a small group of four students during this lab.
• Citric acid H₃C₆H₅O₇ and baking soda NaHCO₃ react to form carbon dioxide CO₂, water H₂O and sodium citrate Na₃C₆H₅O₇
• One mole of citric acid reacts with 3 moles baking soda (sodium bicarbonate) to form 3 moles of carbon dioxide, 3 moles of water and 1 mole of sodium citrate in the following reaction: H₃C₆H₅O₇ + 3NaHCO₃ à 3CO₂ + 3H₂O + Na₃C₆H₅O₇
• Prerequisite topics:
  • Solubility
  • Molar mass
  • Balancing equations
  • Chemical reactions
• Baking soda is partially soluble in water and will have white solids that have not dissolved at the bottom of the beaker in aqueous solution. The reactant citric acid and the product sodium citrate are soluble in aqueous solution. With accurate measurement of the reactants a clear resultant solution affirms that all the baking soda has reacted. A white solid remaining after completion of the reaction indicates the presence of unreacted baking soda.
• Baking soda and citric acid in solid powder or granular form, as a mixture by observation appears unreactive. Adding water to form an aqueous solution initiates a rapid reaction.
• Students first respond to the pre-lab questions to determine the ratio of reactants required, how much of the reactants will need to be weighed for the complete reaction using 0.1 mole of citric acid. Students will then weigh each reactant according to the calculated amounts determined from molar mass and the balanced equation. Both weighed reactants are to be placed in the beaker; the students will observe no reaction.
• Then students will measure 10mL of water and add it to the solid reactants. After the reaction has slowed down students add another 10mL of water to the reaction and swirl until the reaction stops, and there are no remaining solids. The presence of a white precipitate indicates an excess of unreacted baking soda therefore and unbalanced amount of measured reactants.
• Mixing solid citric acid and baking powder: A chemical reaction is not observable with the naked eye. The reaction is occurring at much slower rate than in aqueous solution.
• The chemical reaction can be seen with the naked eye when there is a rapid formation of carbon dioxide being produced, with the displacement of liquid.
• To produce carbon dioxide, the bicarbonate ion needs to come into contact with the acid proton on the citric acid. In the solid state there is very little kinetic energy.
• In solution, the proton transfer can quickly propagate through the hydrogen bond network in water. The greater kinetic energy in liquid rather than solid form increases the number of molecular collisions, and therefore a significantly more rapid rate of reaction occurs that is observable.